The present invention relates to devices and methods for offset and similar printing systems.
More particularly, the present invention relates to devices and methods for screening information from a continuous tone original to produce a halftone image on a photosensitive layer.
In particular, the invention relates to a novel screen, photomechanically or electronically generated, for the preparation of offset or similar printing plates to be used in offset or similar printing of halftone copies of continuous tone originals.
There exists a wide variety of photomechanical screens. Their purpose is to break up the continuous tone information of the original into discrete dots, whose size is related to the optical density of the original. The resultant film, upon processing, produces a xe2x80x9cdotxe2x80x9d image of the original.
In the prior art systems all the dots, which are small enough for the reconstruct into a coherent image are of equal opaque optical density while their relative sizes produce the impression of various grey values. The practical resolution of these screens, and those generated electronically (e.g. via an Electronic Dot Generator Scanner) is usually about 100-200 dots/inch.
In the past, the standard screen utilised an array of symmetrical round or square dots. The details of the original reproduced through such a screen are determined by the arbitrary geometrical centres of the dots in the screen. Further, in the resulting halftone, until 50% dot density one has black dots on a transparent background. An abrupt jump in the tonal scale occurs at 50%, where the four corners of the square dot join at 50%. This discontinuity prevents smooth midtone transitions.
To overcome the latter limitation, an elliptical, diamond, multi-dot or star-like dot pattern is mainly used in modern offset printing. These dot shapes allow a smoother tonal gradation than the square dot, in the 50% area, Two opposite corners of e.g., the diamond shaped dot, join the adjacent dots first at about 40%, while the other two remaining corners join adjacent dots, near 60%. Since the dots join in two steps, a smoother tonal transformation is achieved, This strategy, of having adjacent dots join in multiple steps, is used, for example, in the Double Dot Policrom Screen and the Triplet Dot HRS Beta Screen. The result is smoother, but far from ideal, as the abrupt jump in tonal scales is only attenuated to some degree. Aside from these jumps the image remains discontinuous in the X and Y direction because of the spacing between the dots in both directions which becomes more apparent with a lower screen ruling.
In U.S. Pat. No. 4,768,101 there is disclosed a method of generating a half-tone representation of an image from digital data defining the colour content of pixels arranged in a series of substantially parallel input scan lines.
U.S. Pat. No. 4,700,235 discloses a method and apparatus for producing half-tone printing forms with screens having arbitrary screen angles and screen width.
U.S. Pat. No. 4,833,546 discloses a photomechanical apparatus adapted to print a half-tone picture corresponding to an original continuous tone picture on the basis of tonal information signals obtained by photoelectrically scanning the original continuous tone picture.
U.S. Pat. No. 4,547,812 discloses a method and apparatus for forming high resolution half-tone images.
U.S. Pat. No. 4,543,613 discloses a method for producing half-tone dots in a half-tone plate recording apparatus.
As will be realised none of said patents teach or suggest the device and method of the present invention.
In contradistinction to said prior art photomechanical screens and methods, using the same or involving electronically generating an effective electronic equivalent thereof, the present invention now provides a device for offset and similar printing methods from a continuous tone original to produce a halftone image on a photosensitive layer comprising means for screening said information via a first array of parallely extending lines of effective minimum optical density and a second array of parallely extending lines of effective maximum optical density, said arrays being interposed to form a composite array of substantially parallel, spaced-apart, alternating lines of maximum and minimum optical density, said lines delimiting therebetween zones of effective graduated optical density, the gradient inside each zone progressively varying from low optical density adjacent lines of said first array to high optical density adjacent lines of said second array, the spacing of lines of said first array being between about 100 and 400 lines per inch and wherein the resulting thickness of each reproduced line on a resulting developed photosensitive layer, generated by screening said information via said array, continuously varies as a function of the density of each information point of the original.
In a preferred embodiment of the present invention there is now provided a photomechanical screen for offset and similar printing methods by screening information from a continuous tone original to produce a halftone image on a photosensitive layer, said screen comprising a first array of parallely extending lines of minimum optical density and a second array of parallely extending lines of maximum optical density, said arrays being interposed to form a composite array of substantially parallel spaced-apart, alternating lines of maximum and minimum optical density, said lines delimiting therebetween zones of graduated optical density, the gradient inside each zone progressively varying from low optical density adjacent lines of said first array to high optical density adjacent lines of said second array, the spacing of lines of said first array being between about 100 and 400 lines per inch and wherein the resulting thickness of each reproduced line on a resulting developed photosensitive layer, generated by screening said information via said array, continuously varies as a function of the density of each information point of the original.
The invention also provides a method for generating a halftone image from a continuous tone original comprising exposing a commercial lith or line film to a continuous tone original via a device as hereinbefore defined wherein the resulting thickness of each resulting line generated on said developed lith or line film continuously varies as a function of the density of each information point of the original.
A preferred embodiment of the present method comprises electronically outputting the information of the original onto an unexposed scanner type film, in an electronic output simulating a screen formats defined above, the pattern of this screen being lines spaced 100-400 lines/inch without this film being in contact with a standard photomechanical screen.
In another aspect of the present invention there is also provided a method for generating a halftone image from a continuous tone original comprising outputting the information of the original onto an unexposed scanner-type film, in an electronic output simulating a screen format, the pattern of this screen in a screen format, the pattern of this screen being a first array of parallels extending lines of effective minimum optical density and a second array of parallels extending lines of effective maximum optical density, said arrays being interposed to form a composite array of substantially parallel, spaced apart, alternating lines of effective maximum and minimum optical density, said lines delimiting therebetween zones of effective graduated optical density, the gradient inside each zone progressively varying from low optical density adjacent lines of said first array to high optical density adjacent lines of said second array, the spacing of lines of said first array being between about 400-600 lines per inch and developing in a lith developer containing a hydroquinone developing agent in combination with carbonate, bicarbonate and halide salt together with sodium formaldehyde sulfoxylate and formaldehyde wherein a resulting thickness of each resulting line generated on said scanner type film continuously varies as a function of each information point of the original, but does not decrease in the Dmin region beyond 5 microns width even when the generated line becomes segmented.
The present invention provides a screen capable of producing a final printed, ink on paper, resolution of 100-400 lines/inch, which generates a continuous variation in tonal value in one direction, by using a screen composed of fine lines, 100-400 lines/inch. For example, at 250 lines per inch parallel to the direction of the lines, one would say using prior art concepts and terminology that the segments of a line join to generate an effectively continuous line at about 5-6% dot density. Perpendicular to the direction of the lines, the adjacent lines partially join above 90% dot density.
According to proposed terminology applicable to the novel concept of the present new invention in fact a continuous information straight line from the original, parallely aligned and screened via a line of minimum optical density of the present screening array will be reproduced as a continuous line.
Similarly in the present screening array the width of each line segment is determined by the density of each information point of the original, each such segment having its centre along a line of minimum optical density and extending continuously perpendicularly to the direction of the lines on both sides up to the adjacent bracketing lines of maximum optical density.
Since the xe2x80x9cdotsxe2x80x9d join in two steps outside the operational limits of what would be considered in prior art terms as 5%-90% dot density the tonal transformation is, intrinsically, smooth across the whole tonal range, simply because there are no jumps in tonal scale, within this range, in the direction of the lines. The line screen creates a halftone image by continuously changing the width of the parallel lines, comprising the screen pattern. Thus, unlike xe2x80x9cdotxe2x80x9d based prior art photomechanical or electronic screens, it renders the original continuous tone, in a continuous format, in one direction. For monocolor work, there remains a discontinuity in the direction perpendicular to the lines. However, even in this direction, the image remains black on a white background, until above about 95% dot density. There is no abrupt change to white on black, which is the major cause of disturbance to the eye.
In multicolour printing, where different angles are employed for each of the separations (e.g., here for the line screen, the optimal angles are, 45xc2x0 90xc2x0 105xc2x0 and 165xc2x0). this discontinuity is attenuated, and the result appears to approach a symmetrical continuous tone to the observer.
A possibly more graphic way of explaining the present invention is to state that while the prior art of which applicants are aware is based on symmetrical dot, diamond, elliptical or multi-dot two-dimensional arrays of varying optical density which could be analogised to the peaks and valleys of an egg tray, the present invention is based on parallely extending lines with effective varying optical density of alternating parallely arranged valleys and ridges analogous to a corrugated roof.
In preferred embodiments of the present invention it has been found that using a screening means wherein the distance between lines of said first array are between about 200 and 300 lines per inch with normal rapid access or lith development, will generate a halftone, that with any plate, ink or press, will produce a finer xe2x80x9ccontinuous tonexe2x80x9d reproduction of an original continuous tone, than comparable commercially available screens. This is because, intrinsically, these prior art screens are discontinuous in both the X and Y directions, whereas the line screen of the present invention is continuous in the direction of the line.
As will be seen with reference to FIG. 1 described hereinafter, below 100 lines per inch the eye will perceive the discrete lines of the line screen pattern and thus while Beta Screen Corp. produces a special effect straight line contact screen having a line density of 55 lines/inch, such a lower range cannot be used for the purposes of the present invention.
According to calculations as set forth in example 1 below, much beyond 400 lines/inch, the width of the lines approaches 5xcexc for 50% dot density. Below 50%, well above the Dmin value, the line has to be less than 5xcexc, which is too small for graphic art films, so the result is 100% transparent. Over 50%, well below Dmax value, the distance between lines is less than 5xcexc. Again, graphic art films cannot handle it, and silver image will be joined over this distance, creating solid black. Thus for most uses contemplated the effective upper range should not exceed 400 lines per inch.